Traditionally, powered wheelchairs have two drive wheels at the rear and two castor wheels at the front. Each of these drive wheels may be driven by its own motor and, by driving the drive wheels at different speeds, the chair can be turned. These chairs drive like a car in that they pivot about the rear of the chair (in a similar manner to the way a car pivots about its rear wheels). Accordingly, such motorized wheelchairs are generally adapted for movement either along a straight line or along a steered curved arc, somewhat in the same manner as an automobile. They require a significant turning radius and, as with a car, many manoeuvres must be executed backwards. Further, these chairs tend to lose traction on downward slopes since the rear drive wheels tend to become unloaded.
If it is desired to realign the wheelchair for movement in a new direction, it is typically necessary to go through complex turning manoeuvres, somewhat similar to the three point turn utilized on occasion in operating an automobile. The manoeuvres require a significant amount of space and many tight spaces must be approached backwards in a manner similar to a car reversing into a parking spot. These complex manoeuvres are tiresome and often difficult for disabled persons to carry out. While there are some powered chairs where the powered drive wheels are at the front, these chairs also require a large turning radius.
Several wheelchairs have been designed with the drive wheels positioned near the centre of the chair. Generally, these chairs, which are known as mid-wheel drive chairs, operate with the two drive wheels and either a front pair or a rear pair of wheels in contact with the ground. Thus for stability, the drive wheels must be positioned either in front of or behind the centre of gravity of the chair so that the chair is balanced on the drive wheels and either the front wheels or the rear wheels. The other pair of wheels act as anti-tippers to prevent the chair from tipping over on slopes, or during acceleration or deceleration. These anti-tipping wheels are generally held above the surface and are not in contact with the ground. These mid-wheel drive chairs are garnering a reputation as being unstable since, in operation, the chairs lurch considerably as the chair tips onto its anti-tipping wheels. Stability is improved by moving the drive wheels farther away from the centre of gravity of the chair. At the same time, however, the manoeuvrability of the chair decreases as the drive wheels are placed farther away from the centre of gravity of the chair, since the rotation no longer takes place at the centre of the chair (i.e. at the centre of gravity).
In U.S. Pat. No. 5,445,233 (Fernie et al.), the applicants disclosed a novel mid-wheel design for a motorized wheelchair, with the drive wheels directly underneath the chair's centre of gravity. This wheelchair utilized a pair of drive wheels which were centrally mounted beneath the seat of the chair and four free running stabilizing (caster) wheels which were positioned at each of the four corners of the base of the chair. In order to stabilize the chair so as to travel over uneven surfaces, the drive wheels were rotatably mounted on a shaft at a fixed position beneath the seat of the chair. The free running wheels were mounted on a collar so as to be vertically movably mounted on the shaft. The free running wheels were urged into contact with the ground via a spring mounted around the shaft. Accordingly, for example, if the wheelchair of Fernie et al were travelling in a forward direction and encountered uneven terrain, the front wheels could pivot upwardly or downwardly about the shaft while ensuring that the drive wheels remained in contact with the ground. The spring would continually urge the free running wheels into engagement with the ground to stabilize the wheelchair and thereby prevent the wheelchair from tipping over.
The motorized mid-wheel wheelchair design disclosed by the applicants in U.S. Pat. No. 5,445,233 could be operated from a standing start in any desired direction without the necessity of a series of forward and reverse movements. By driving the motors of the two drive wheels in opposite directions (at the same speed) the drive unit could be rotated to face a desired direction, for instance the direction indicated by a joystick, and then operated to move from a standing start in that direction. During the rotation of the drive unit the clutch could be disengaged so that the orientation of the chair does not change (alternatively the clutch could remain engaged so that the chair rotates with the drive unit). This patent did not disclose a clutch design.
While the ability to move the chair in any desired direction seems beneficial, in work conducted by the applicant, this has proven not to be the case. Human beings tend to think and operate along Cartesian coordinates. Accordingly, when they are seated in a chair, they have trouble controlling diagonal motion. Thus the ability to permit the chair to rotate to any desired angle provides little or no advantage to a person operating a chair, and adds considerably to the complexity of the clutch mechanism. These clutch complexities further require additional parts which are subject to wear and tear and potential failure. These additional parts also add substantially to the cost of the wheelchair, thereby restricting the ability of some disabled people to acquire the wheelchair.